The present invention relates to monitoring a patient's cardiopulmonary function and blood condition, and particularly to a method and apparatus for monitoring the performance of a patient's heart and lungs during and following thoracic surgery.
Cardiac output means the volume rate at which the heart pumps blood. The cardiac output is important to a clinician as an indication of how well a person's heart is able to function. Using conventional techniques and available apparatus it is difficult and costly to evaluate a patient's cardiac output and respiratory efficiency, and the process cannot be performed as quickly as desirable. Following surgery, and particularly heart surgery, it would be advantageous to be able to determine quickly and frequently how well a patient's heart and lungs are performing in delivering oxygenated blood to the patient's tissues in relation to how quickly the same tissues are removing the oxygen with which they are being supplied.
To accurately determine the efficiency of the heart and lungs relative to the body's need for oxygen it is advantageous to analyze the percentage of oxygen saturation of hemoglobin (hereinafter simply called oxygen saturation) in mixed venous blood, as found in the pulmonary artery. It is also advantageous, and even more accurate in assessing a person's cardiac function, to compare the oxygen saturation of blood in the pulmonary artery with oxygen saturation in freshly oxygenated blood, as found in the aorta. In the past it has been necessary to chemically analyze drawn samples of blood to evaluate blood oxygen saturation levels accurately. Such analysis is costly, and it has usually not been practical to obtain such blood samples.
It is known to approximately evaluate the percentage of saturation of hemoglobin by oxygen (oxygen saturation) of the blood in peripheral tissues by use of an external sensor involving a light source and a receptor and evaluating the blood's effect on transmission of light through tissues immediately below the skin of an external part of a patient's body, such as the ear lobe, nose or finger. Such external sensors, known as oximeters, are available, for example, from Nellcor of Pleasanton, Calif. A measurement obtained using such a device can be used to evaluate major changes in arterial blood oxygenation, but does not provide enough information for determining a patient's cardiac output, since it does not provide enough information regarding oxygen extraction or utilization by tissues and thus is not a good enough tool for valid evaluation of a patient's cardiopulmonary function during and after cardiac surgery.
Catheters equipped with light-emitting and receiving sensors can be placed within the blood flowing through the pulmonary artery itself. Such sensors, when thus residing in the bloodstream, can be used to measure oxygen saturation in mixed venous blood. This is a relatively invasive procedure, however, and can be used for only a limited time, after which the sensors would become covered with protein deposits from the blood and would thereby lose their sensitivity.
There is no currently available implantable device that remains separate from and outside the flow of blood for measuring oxygen saturation in blood without blood samples having to be drawn for analysis.
What is desired, then, is to be able to measure various blood characteristics, such as to analyze the level of blood oxygen saturation and the levels or concentrations of other blood components, including potassium, lactate, glucose, pH, hemoglobin or hematocrit (red blood cell volume percentage), to be able to determine those aspects of blood condition rapidly and repeatedly during and following a surgical procedure and for a period of time thereafter, and to do so at a cost which is less than the cost for repeatedly drawing and chemically analyzing or microscopically examining blood samples. Additionally, it is desirable to be able to monitor such blood characteristics over a long term in some persons.